This is a common question.
“How can I set exact/precise sizes in Maya?”

A simple way:

• Select the object
• Make sure you have equal scale values.
  For example with Modify->Freeze Transformations (Option Box, use only scale).
• Press Ctrl+T (Universal Manipulator)
• Click on the number which you want to define (that is a distance value in cm).
  It’s converted to a scale value as you can see in the Channel Box.
• In Channel Box: copy the Scale value into the other two.

Cheers, D